The PNG File Format

The PNG file format is a very versatile and flexible file type. The colour depth may vary from one single bit upto 48bit. In 48bit mode each colour channel has not allocated 8bit , but 16bit, resulting in 2¹⁶=65,536 shades for colour channel. Nevertheless this enormous colour depth is only used in highly specialised scientific applications.

Common computer displays and browser are able to handle colour depths up to 24bit resulting in 16,777,216 possible colours. The PNG file format allows to add an alpha-Channel of 8bit to the RGB colour channels. This alpha channel adds transparency to an image. The rate of transparency can be set in 256 steps from fully transparent to fully opaque.

However, Internet Explorer with its lacking standards' support, is not able to display 32bit PNG images correctly. This browser only supports full transparency in indexed 8bit PNG images. Luckily enough there exists a hack to enable Internet Explorer displaying PNG images with alpha transparency. This hack makes use of one of Internet Explorer's very own filters, the AlphaImageLoader, which enables alpha-channels for images. Erik Arvidsson describes, how this workaround solution is applied.

PNG images can also be losslessly compressed resulting in considerable reduction of file size. How resounding this reduction may become I demonstrate in a small chart based on an image [PNG, 4096 x 4096 pixels, 59,859 bytes] containing all 16,7 million colours of the 24bit RGB colour space, created by Paul Schmidt of the PNG group [PNG, 1,1MB!].